fix: move hooks before early return in BoardView, remove unused attachmentMode prop

Fixed React hooks rules violation where useState and useCallback were
called after a conditional return in BoardView. Removed unused
attachmentMode prop from AttachmentSection (can be re-added when file
dialog is wired up).

src/components/card-detail/AttachmentSection.tsx

@@ -6,7 +6,6 @@ import type { Attachment } from "@/types/board";
 interface AttachmentSectionProps {
   cardId: string;
   attachments: Attachment[];
-  attachmentMode: "link" | "copy";
 }
 
 export function AttachmentSection({

src/components/card-detail/CardDetailModal.tsx

@@ -24,9 +24,6 @@ export function CardDetailModal({ cardId, onClose }: CardDetailModalProps) {
     cardId ? s.board?.cards[cardId] ?? null : null
   );
   const boardLabels = useBoardStore((s) => s.board?.labels ?? []);
-  const attachmentMode = useBoardStore(
-    (s) => s.board?.settings.attachmentMode ?? "link"
-  );
   const updateCard = useBoardStore((s) => s.updateCard);
 
   const open = cardId != null && card != null;
@@ -82,7 +79,6 @@ export function CardDetailModal({ cardId, onClose }: CardDetailModalProps) {
                 <AttachmentSection
                   cardId={cardId}
                   attachments={card.attachments}
-                  attachmentMode={attachmentMode}
                 />
               </div>
             </div>